The great Austrian conductor and early-music pioneer Nikolaus Harnoncourt announced his retirement this weekend, as he celebrated his 86th birthday.

Mr. Harnoncourt — who founded the Concentus Musicus Wien in 1953, teamed up with Gustav Leonhardt to record nearly 200 of Johann Sebastian Bach’s cantatas for Teldec, and became a much-sought-after conductor of later music as well — announced his retirement in a handwritten note, copies of which were placed in the programs at a Concentus Musicus concert in Vienna.

“Dear audience,” Mr. Harnoncourt wrote in the letter, which was in German. “My bodily strength requires me to cancel my future plans.”

He wrote of the “unbelievably deep relationship has developed between us on the stage and you in the hall,” added that “we have become a happy community of pioneers,” and urged audiences to remain faithful to that spirit.